Tandem compressors with pulse width modulation suction valve

ABSTRACT

A refrigerant system is provided with tandem compressors. A tandem compressor arrangement includes at least two compressors operating in parallel and having at least one common manifold. A control may operate the compressors either simultaneously, or in some predetermined sequence to provide control over refrigerant system capacity. At least one of the tandem compressors is provided with a pulse width modulation control on a suction line. In this manner, the amount of refrigerant, compressed by the compressor, can be precisely controlled to exactly meet thermal load demands in the conditioned space.

BACKGROUND OF THE INVENTION

This application relates to a refrigerant system incorporating tandemcompressors and pulse width modulation control on a suction line leadingto at least one of the tandem compressors.

Refrigerant HVAC&R systems typically include a compressor delivering acompressed refrigerant from a compressor discharge port to a condenser,and then passing the refrigerant from the condenser to an expansiondevice, an evaporator, and finally back to the compressor suction portthroughout a closed-loop circuit. The thermal load demand on therefrigerant system may vary and generally depends on indoor and outdooroperational environments, thermal load generation in a conditioned spaceand fresh air circulation requirements. At times, there may be a needfor a higher system cooling capacity, and hence higher flow ofrefrigerant circulating throughout the refrigerant system is required.At other times, a lower cooling capacity, and consequently lowerrefrigerant flow, may be adequate to maintain the conditioned spacewithin the comfort zone. To provide sufficient means of refrigerant flowcontrol, some refrigerant systems utilize tandem compressors to provideunloading capability by switching off one of the tandem compressors tomatch the system capacity to the thermal load in the conditioned space.In such systems, two or more compressors may simultaneously deliver acompressed refrigerant to a downstream heat exchanger, such as acondenser. Typically, individual discharge lines communicate with thedischarge ports of the tandem compressors. These discharge lines arethen merged into a single discharge manifold connected to a condenser.Similarly, individual suction lines communicate with the suction portsof the tandem compressors. These suction lines emerge from a singlesuction manifold connected to a line extending from the evaporator exit.On the other hand, tandem compressor systems are known, wherein separatecondensers are associated with each of the compressors, while thecompressors are still connected to the same evaporator. Analogously,tandem compressor systems may be connected to separate evaporators,while still communicating to the same condenser. The last twoconfigurations are typically utilized when either condensers orevaporators are associated with separate indoor or outdoor environmentsthat may have different operational characteristics.

A control for a typical tandem compressor system will operate one, orseveral compressors, depending on system thermal load. Thus, thecompressors can be controlled to provide discrete steps in systemcapacity. Also, as known, tandem compressor arrangements may includepressure and oil equalization lines to prevent oil pumpout fromcompressors and improve reliability.

One method of providing finer control over the capacity of a refrigerantsystem is the use of pulse width modulation controls for a refrigerantsystem compressor. With such a control, a suction pulse width modulationvalve is cycled at a predetermined rate between on and off positions toprevent and then allow the flow of refrigerant to the compressor. Sincethe valve is cycled between open and closed positions, the throttling orany other losses are practically eliminated. In this manner, the amountof refrigerant compressed by the compressor can be finely tailored to adesired capacity, while maintaining efficient system operation. Whilepulse width modulation controls are known in refrigerant systems, theyhave not been incorporated into tandem compressor systems.

SUMMARY OF THE INVENTION

In a disclosed embodiment of this invention, two or more tandemcompressors are operated in a refrigerant system. A suction line leadingto at least one of two compressors is provided with a suction valvehaving a pulse width modulation control. Thus, the provided capacity ofthat compressor can be finely tailored to thermal load demands in anenvironment to be conditioned. In one embodiment, only one of thecompressors is provided with such a suction valve controlled by pulsewidth modulation. In another embodiment, the suction pulse widthmodulation valve is provided on a manifold leading to each of thecompressors.

In various embodiments, the compressors may deliver refrigerant toseparate condensers, while still connected to a single evaporator, ormay receive refrigerant from separate evaporators, while stillcommunicating compressed refrigerant to a single condenser. In otherembodiments, the tandem compressor refrigerant system may be providedwith an economizer cycle and/or a bypass feature to achieve even moreflexible control over supplied capacity.

These and other features of the present invention can be best understoodfrom the following specification and drawings, the following of which isa brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first schematic refrigerant system.

FIG. 2 shows a second schematic.

FIG. 3 shows a third schematic.

FIG. 4 shows a fourth schematic.

FIG. 5 shows a capacity chart for the FIG. 1 schematic.

FIG. 6 shows a fifth schematic.

FIG. 7 shows a sixth schematic.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a refrigerant system 20 having two compressors 22 and 24operating in tandem to compress refrigerant and deliver the refrigerantto a common discharge manifold 28. A single condenser 30 receivesrefrigerant from the common discharge manifold 28 and delivers thatrefrigerant to an expansion device 32. An evaporator 34 is positioneddownstream of the expansion device 32. Refrigerant from the evaporator34 passes into a common suction manifold 26 and back to the compressors22 and 24. As is well known in the art, a control 38 for the systemoperates to drive one or both of the compressors 22 and 24 to provide adesired capacity for the refrigerant system 20. The compressors 22 and24 can be of the same size, or can be of different sizes. The controland operation of a tandem compressor refrigerant system are, as known inthe art, and thus no further description is deemed necessary. What isinventive is the provision of a suction valve 36 having pulse widthmodulation control by the control 38, or by a dedicated controller.Thus, as known, the control 38 cycles the valve 36 at a predeterminedrate between an open (normally fully open position) and closed position(normally either fully closed or nearly closed position) to control theflow of refrigerant to the compressors 22 and 24. In this manner, theamount of refrigerant delivered to the compressors 22 and 24 can befinely tailored to achieve precise capacity values, no matter how manycompressors are operated at a particular moment of time. Since the pulsewidth modulation valve 36 is cycled from an open to closed position,there are minimal throttling or other losses that are associated withthe valve 36. The cycling rate and time interval for the valve 36 tostay in an open position are determined by the capacity to be deliveredto a conditioned environment, reliability requirements, allowablecomfort zone parameter variations and refrigerant system thermalinertia. While two tandem compressors are shown, refrigerant systems areknown with three, four or even higher number of tandem compressors. Thepresent invention thus provides a very powerful and efficient means ofachieving a precisely tailored refrigerant system capacity from a tandemcompressor configuration, while minimizing temperature and humidityvariation in the conditioned space, improving occupant's comfort andreducing power consumption.

FIG. 2 shows another embodiment 220, wherein the tandem compressors 122and 124 again deliver refrigerant to a common discharge manifold 28 anda downstream condenser 30. This system configuration is distinct in thatthe refrigerant branches to two separate expansion devices 132, and twoseparate evaporators 134A and 134B. Such an arrangement is typicallyprovided when the evaporators are serving differentconditioned/refrigerated zones and having different operationalcharacteristics. Thus, there is no common suction manifold deliveringrefrigerant to each of the compressors 122 and 124. In this embodiment,only one of the compressors, the compressor 122, has a suction pulsewidth modulated valve 136, mounted on its suction line leading to thatcompressor, which is controlled by a pulse width modulation control 138.As above, the control 138 can be a separate control or integrated into asystem control for the refrigerant system 220. Again, when finetailoring of the capacity delivered by the refrigerant system 220, andthe evaporator 134A in particular, is desired, the compressor 122 wouldbe operated, as it is able to provide precise refrigerant flow controlby its associated pulse width modulation suction valve 136. Once again,although only two tandem compressors are show in FIG. 2, with only oneof them having an associated pulse width modulation valve, a highernumber of tandem compressors, with several of them having associatedpulse width modulation valves, would be within the scope of theinvention.

FIG. 3 shows yet another embodiment 221, wherein each of the compressors22 and 24 deliver refrigerant to separate condensers 230 and 232, whilestill receiving refrigerant from the same evaporator 34. Such anarrangement would be typically used when the condensers reject heat intodifferent environments (such as, for instance, indoors and outdoors) andhave different operational characteristics. The condensers 230 and 232deliver refrigerant to individual expansion devices 233, and therefrigerant flows are then combined before returning to the evaporator34, and the common suction manifold 26. Again, a pulse width modulationvalve 36 is mounted on the suction line leading to the tandemcompressors 22 and 24 and controls refrigerant flow through therefrigerant system 221, no matter how many compressors are in operation.Similar to the FIG. 2 embodiment, the pulse width modulation valve 36can be associated with only one of the compressors 22 or 24 and controlthe refrigerant flow through that particular compressor and anassociated condenser, if desired (see further explanation below). Onceagain, this embodiment can be extended to any number of tandemcompressors.

FIG. 4 shows a refrigerant system 222. This system is somewhat similarto the FIG. 1 system; however, the pulse width modulation valve 136 ispositioned downstream of the common suction manifold 26, and on asuction line delivering refrigerant only to the compressor 122. Withthis arrangement, it may be desirable to only operate one of thecompressors 122 and 124 at any instant of time, since compressor oilsumps of the two compressors 122 and 124 would be at a considerablepressure differential, when the pulse width modulation control isactivated for the valve 136. Otherwise, cross-leakage between thecompressors, oil pumpout and reliability problems may take place. Again,the compressor 122 is capable of providing precise control to thecapacity provided by the refrigerant system 222. Any number of tandemcompressors may have the associated pulse width modulated valves in thisarrangement as well.

A tailored refrigerant system capacity achieved by a pulse widthmodulation valve of FIG. 1 is illustrated in FIG. 5. As shown in thisFigure, if full capacity is required then two tandem compressors operatetogether with pulse width modulation valve fully open. When the capacityrequirements are reduced, the pulse width modulation valve will cyclebetween open and closed positions. As more capacity reduction isrequired, the valve would stay in a closed position for longer periodsof time than in an open position. When the capacity requirements aresufficiently low, then one of the tandem compressors is shut off, andthe pulse width modulation valve cycle is altered once again to be mostof the time in an open position. As the capacity requirements arereduced even further, the system continues to operate with onecompressor turned off, and the cycle of the pulse width modulation valveadjusted, such that the pulse width modulation valve stays in a closedposition for longer periods of time. The most appropriate time when thecontroller turns off one of the tandem compressors is determined andpredominately based on the system capacity requirements, while it can beadjusted further to take system efficiency and reliability intoconsideration. While FIG. 5 illustrates adjustments in the pulse widthmodulation valve position for schematic shown in FIG. 1, it can beapplied in a similar fashion to other embodiments of this invention asshown in other Figures.

FIG. 6 shows enhancement features that can be incorporated into any ofthe above schematics, in a system 300. The tandem compressors 302 and304 are similar to the above embodiments, and the compressor 302 has anassociated pulse width modulation valve 303 with a pulse widthmodulation control 305. However, even more flexibility in capacitycontrol is provided in this arrangement. A condenser 306 receivesrefrigerant from the tandem compressors 302 and 304, and delivers it toa liquid line. A portion of refrigerant is tapped from the liquid linedownstream of the condenser 306, and the tapped refrigerant passesthrough an economizer expansion device 310, where it is expanded to alower pressure and temperature, and into an economizer heat exchanger312 for the heat transfer interaction with the refrigerant circulatingthrough the main circuit. As is known, the refrigerant passing through atap line 308 subcools the refrigerant in a liquid line 314 flowing intothe main expansion device (not shown). This provides a greater thermalpotential for the refrigerant entering an evaporator (also, not shown)and consequently enhances refrigerant system cooling capacity and/orefficiency. The use of an economizer cycle is known in the art. Althoughthe flow from the tap line 308 and the liquid line 314 are shown passingthrough the economizer heat exchanger 312 in the same direction, this isfor illustration simplicity only. In practice, they are preferablyflowing in a counterflow arrangement. The refrigerant from the tap line308 is then flown through a return line 316, having a shutoff valve 318,and a vapor injection line 320. As is known in the art, the vaporinjection line 320 would inject the returned refrigerant from the line316 to an intermediate compression point in the economized compressor302. It should be noted that the shutoff valve 318 may not be need, ifthe economizer expansion device 310 is equipped with a shutoffcapability.

In addition, and as shown schematically in FIG. 5, a return line 330,shown in phantom, can also return refrigerant to the second compressor304, if the compressor 304 is of an economized type as well. It shouldbe noted that the compressors 302 and 304 may not necessarily share thesame economizer branch components such as economizer heat exchanger 312and economizer expansion device 310.

A bypass valve 322 may be opened to selectively bypass at least aportion of partially compressed refrigerant back from the compressor302, through the vapor injection line 320, and a bypass line 324 to asuction line. Typically (but not always), a bypass function is utilizedin a non-economized mode of operation. Further, the compressor 304 maybe also equipped with a bypass function. Again, the use of the bypassfor compressor unloading is as known in the art. However, the additionof the bypass and/or economizer function into a tandem compressor systemhaving at least one compressor provided with a pulse width modulationcontrol allows for even more flexible control over the providedcapacity. Since an economizer and bypass functions offer two additionaldiscrete steps of capacity control, the pulse width modulation techniqueoffers precision control for refrigerant system operation within theeconomizer stage (when an economizer circuit is engaged) and bypassstage (when the bypass function is activated). As a result, superioraccuracy control is provided in the conditioned space and efficiencyboost in the refrigerant system operation is achieved. Once again, morethan a single tandem compressor can be equipped with economizer andbypass features (whether or not sharing the economizer branch componentssuch an economizer heat exchanger and economizer expansion device) andassociated with the pulse width modulation control. Also, the economizerand bypass function do not need to be combined with each other. Forexample, only an economizer feature or only a bypass feature can beassociated with a particular compressor. Further, as known in the art,it might be desirable to provide a small leak through a pulse widthmodulation valve (or a small bypass around the valve), when the valve isin a closed position to prevent the compressor operating in a vacuum.

It should be pointed out that while the present invention providesillustration for only two tandem compressors, as known in the art, morethan two tandem compressors can be connected together in a tandemconfiguration. Also each compressor shown in the above Figures canrepresent a bank of compressors connected together in tandem andproviding a nested arrangement. Within this nested arrangement, anycompressor can be equipped with a pulse width modulated valve. Forinstance, a two-level nested tandem compressor system, incorporating twocompressor banks 422 and 424 and suction modulation valves 436, is shownin FIG. 7. It also should be pointed out that, in general, the pulsewidth modulation valve can be associated with any compressor in thesystem or with any compressor type, such as a scroll compressor, arotary compressor, a reciprocating compressor, a screw compressor, etc.Furthermore, the described refrigerant systems can be of a singlecircuit configuration or can be a part of a multi-circuit arrangement.In multi-circuit configurations, only a single circuit may be equippedwith tandem compressors having a suction pulse width modulation valve ormultiple circuits may have tandem compressors and an associated suctionpulse width modulation valve.

Although a preferred embodiment of this invention has been disclosed, aworker of ordinary skill in this art would recognize that certainmodifications would come within the scope of this invention. For thatreason, the following claims should be studied to determine the truescope and content of this invention.

1. A refrigerant system comprising: at least two compressors operatingin parallel to compress refrigerant and deliver refrigerant downstreamto at least one condenser, at least one expansion device positioneddownstream of said at least one condenser, and at least one evaporatorpositioned downstream of said at least one expansion device; andrefrigerant returning from said at least one evaporator through at leastone suction line to said at least two compressors, and a suction valveprovided between said evaporator and at least one of said at least twocompressors, said suction valve being provided with a pulse widthmodulation control to control the amount of refrigerant being deliveredto said at least one of said at least two compressors.
 2. Therefrigerant system as set forth in claim 1, wherein at least two of saidat least two compressors have at least one common manifold.
 3. Therefrigerant system as set forth in claim 2, wherein said at least onecommon manifold is a suction manifold.
 4. The refrigerant system as setforth in claim 2, wherein said at least one common manifold is adischarge manifold.
 5. The refrigerant system as set forth in claim 1,wherein at least two of said at least two compressors deliverrefrigerant to a single condenser.
 6. The refrigerant system as setforth in claim 1, wherein a single expansion device receives refrigerantfrom at least two of said at least two compressors.
 7. The refrigerantsystem as set forth in claim 1, wherein a single evaporator deliversrefrigerant to a suction manifold returning refrigerant to at least twoof said at least two compressors.
 8. The refrigerant system as set forthin claim 7, wherein said suction valve is provided on said suctionmanifold, and thus regulates the flow of refrigerant to at least two ofsaid at least two compressors.
 9. The refrigerant system as set forth inclaim 7, wherein said suction valve is provided downstream of saidsuction manifold and on a line delivering refrigerant to at least one ofsaid at least two compressors.
 10. The refrigerant system as set forthin claim 1, wherein there are at least two of said condensers separatelyreceiving refrigerant from said at least two compressors.
 11. Therefrigerant system as set forth in claim 10, wherein each of said atleast two condensers is provided with a separate expansion device. 12.The refrigerant system as set forth in claim 1, wherein there are atleast two evaporators, and said at least two evaporators separatelyreturning refrigerant to said at least two compressors.
 13. Therefrigerant system as set forth in claim 12, wherein at least twoexpansion devices are provided and separately pass refrigerant to saidat least two evaporators.
 14. The refrigerant system as set forth inclaim 1, wherein at least one of said at least two compressors isprovided with a bypass function.
 15. The refrigerant system as set forthin claim 14, wherein at least one of said at least two compressors isprovided with a bypass function and said at least one compressor isassociated with said suction valve.
 16. The refrigerant system as setforth in claim 1, wherein at least one of said at least two compressorsis provided with an economizer cycle.
 17. The refrigerant system as setforth in claim 16, wherein at least one of said at least two compressorsis provided with an economizer cycle and said at least one compressor isassociated with said suction valve.
 18. The refrigerant system as setforth in claim 16, wherein more than one of said at least twocompressors is provided with an economizer cycle and these economizedcompressors are sharing at least one of the economizer branchcomponents.
 19. The refrigerant system as set forth in claim 1, whereinat least one of said at least two compressors is a compressor bank andsaid suction valve is associated with at least one compressor in thebank.
 20. The refrigerant system as set forth in claim 1, wherein atleast one of said at least two compressors is selected from a set of ascroll compressor, a rotary compressor, a screw compressor, and areciprocating compressor. 21.-39. (canceled)